The present invention relates to a method for photopolymerizing a monomer mixture to form a lens, wherein the monomer mixture may include a UV-absorbing compound and a tint and is exposed to a light source including light in the visible region of the spectrum.
Lenses such as contact lenses or intraocular lenses may include a UV absorbing agent in the lens to absorb light in the ultraviolet region of the spectrum, more particularly, to absorb light in the region of about 200 to 400 nm and, especially, about 290 to 400 nm. Representative UV absorbing materials for such lens applications are described in U.S. Pat. No. 4,304,895 (Loshaek), U.S. Pat. No. 4,528,311 (Beard et al.) and U.S. Pat. No. 4,719,248 (Bambury et al.).
Such lenses may also include a tint. The tint may be relatively rich in hue, so as to change or at least enhance the color of the iris when the lens is placed thereon. Alternately, the tint may be relatively light in hue, so that it does not change or enhance the color of the iris but does facilitate handling of the lens by a user; a representative xe2x80x9cvisibility tintxe2x80x9d for contact lenses is described in U.S. Pat. No. 4,997,897 (Melpolder).
Generally, such lenses are formed by free radical polymerization of a monomer mixture including desired lens-forming monomers, usually in the presence of heat (thermal polymerization) or a light source (photopolymerization). One particular method for producing contact lenses involves thermal polymerization of the initial monomeric mixture in tubes in a heated water bath to provide rod-shaped articles, which rods are then cut into buttons, the buttons then being lathed into contact lenses; such methods for forming lenses including a UV absorbing agent are illustrated in the aforementioned U.S. Pat. No. 4,304,895 (Loshaek) and U.S. Pat. No. 4,528,311 (Beard et al.). Other methods involve casting the lenses directly in molds, wherein the monomer mixture is charged to the mold and polymerized by exposure to ultraviolet radiation.
Among photopolymerization processes, UV curing (i.e., exposure of the monomer mixture to radiation mainly in the ultraviolet region) of the monomer mixtures has proved very effective. However, for lenses including a UV absorbing agent, problems are encountered when attempting to cure the monomer mixtures since this agent absorbs UV light, thus diminishing the amount of UV light available to effect polymerization and resulting in ineffective or uneven curing of the monomer mixture.
It is also possible to effect photopolymerization using a light source also including light in the visible region of the spectrum, although light in this region is generally less efficient in effecting polymerization of conventional lens-forming monomer mixtures than UV curing. U.S. Pat. 4,719,248 (Bambury) reports successful polymerization of contact lens compositions including a UV absorbing agent by exposure of the monomer mixture to visible light. However, it has been found that the methods illustrated in the Bambury patent could not effectively polymerize monomer mixtures for contact lenses that included, in addition to the UV absorbing agent, a tinting agent.
Accordingly, it would be desirable to provide a method whereby lenses including both a UV absorbing agent and a tinting agent can be effectively photopolymerized by free radical polymerization. The present invention provides such a method and solves the aforementioned problems.
The invention provides a method for photopolymerizing a monomer mixture to form a lens comprising charging to a mold a monomer mixture including lens-forming monomers, and exposing the monomer mixture in the mold to a light source including light in the visible region of the spectrum. The method is useful for monomer mixtures that include a UV-absorbing compound and a tinting agent. Preferably, the monomer mixtures include a polymerization initiator including a phosphine oxide moiety.
The monomer mixtures employed in the invention include conventional lens-forming monomers, UV absorbing agents and tinting agents.
The lens-forming monomers are monomers that are polymerizable by free radical polymerization, generally including an activated unsaturated radical, and most preferably an ethylenically unsaturated radical. (As used herein, the term xe2x80x9cmonomerxe2x80x9d denotes relatively low molecular weight compounds that are polymerizable by free radical polymerization, as well as higher molecular weight compounds also referred to as xe2x80x9cprepolymersxe2x80x9d, xe2x80x9cmacromonomersxe2x80x9d, and related terms.)
An especially preferred class of lens-forming monomers are those that form hydrogel copolymers. A hydrogel is a crosslinked polymeric system that can absorb and retain water in an equilibrium state. Accordingly, for hydrogels, the monomer mixture will typically include a hydrophilic monomer. Suitable hydrophilic monomers include: unsaturated carboxylic acids, such as methacrylic and acrylic acids; acrylic substituted alcohols, such as 2-hydroxyethylmethacrylate and 2-hydroxyethylacrylate; vinyl lactams, such as N-vinyl pyrrolidone; and acrylamides, such as methacrylamide and N,N-dimethylacrylamide.
Another preferred class of lens-forming monomers are those that form silicone hydrogel copolymers. Such systems include, in addition to a hydrophilic monomer, a silicone-containing monomer. One suitable class of silicone containing monomers include known bulky, monofunctional polysiloxanylalkyl monomers represented by Formula (I): 
wherein:
X denotes xe2x80x94COOxe2x80x94, xe2x80x94CONR4xe2x80x94, xe2x80x94OCOOxe2x80x94, or xe2x80x94OCONR4xe2x80x94 where each where R4 is H or lower alkyl; R3 denotes hydrogen or methyl; h is 1 to 10; and each R2 independently denotes a lower alkyl or halogenated alkyl radical, a phenyl radical or a radical of the formula
xe2x80x94Si(R5)3
wherein each R5 is independently a lower alkyl radical or a phenyl radical. Such bulky monomers specifically include methacryloxypropyl tris(trimethylsiloxy)silane, pentamethyldisiloxanyl methylmethacrylate, tris(trimethylsiloxy)methacryloxy propylsilane, methyldi(trimethylsiloxy)methacryloxymethyl silane, 3-[tris(trimethylsiloxy)silyl] propyl vinyl carbamate, and 3-[tris(trimethylsiloxy)silyl] propyl vinyl carbonate.
Another suitable class are multifunctional ethylenically xe2x80x9cend-cappedxe2x80x9d siloxane-containing monomers, especially difunctional monomers represented Formula (II): 
wherein:
each Axe2x80x2 is independently an activated unsaturated group;
each Rxe2x80x2 is independently are an alkylene group having 1 to 10 carbon atoms wherein the carbon atoms may include ether, urethane or ureido linkages therebetween;
each R8 is independently selected from monovalent hydrocarbon radicals or halogen substituted monovalent hydrocarbon radicals having 1 to 18 carbon atoms which may include ether linkages therebetween, and
a is an integer equal to or greater than 1. Preferably, each R8 is independently selected from alkyl groups, phenyl groups and fluoro-substituted alkyl groups. It is further noted that at least one R8 may be a fluoro-substituted alkyl group such as that represented by the formula:
xe2x80x94Dxe2x80x2xe2x80x94(CF2)sxe2x80x94Mxe2x80x2
xe2x80x83wherein:
Dxe2x80x2 is an alkylene group having 1 to 10 carbon atoms wherein said carbon atoms may include ether linkages therebetween;
Mxe2x80x2 is hydrogen, fluorine, or alkyl group but preferably hydrogen; and
s is an integer from 1 to 20, preferably 1 to 6.
With respect to Axe2x80x2, the term xe2x80x9cactivatedxe2x80x9d is used to describe unsaturated groups which include at least one substituent which facilitates free radical polymerization, preferably an ethylenically unsaturated radical. Although a wide variety of such groups may be used, preferably, Axe2x80x2 is an ester or amide of (meth)acrylic acid represented by the general formula: 
wherein X is preferably hydrogen or methyl, and Y is xe2x80x94Oxe2x80x94 or xe2x80x94NHxe2x80x94. Examples of other suitable activated unsaturated groups include vinyl carbonates, vinyl carbamates, fumarates, fumaramides, maleates, acrylonitryl, vinyl ether and styryl. Specific examples of monomers of Formula (II) include the following: 
wherein:
d, f, g, h and k range from 0 to 250, preferably from 2 to 100; and
Mxe2x80x2 is hydrogen or fluorine.
A further suitable class of silicone-containing monomers includes monomers of the Formulae (IIIa) and (IIIb):
Exe2x80x2(*D*A*D*G)a*D*A*D*Exe2x80x2; orxe2x80x83xe2x80x83(IIIa)
Exe2x80x2(*D*G*D*A)a*D*G*D*Exe2x80x2;xe2x80x83xe2x80x83(IIIb)
wherein:
D denotes an alkyl diradical, an alkyl cycloalkyl diradical, a cycloalkyl diradical, an aryl diradical or an alkylaryl diradical having 6 to 30 carbon atoms;
G denotes an alkyl diradical, a cycloalkyl diradical, an alkyl cycloalkyl diradical, an aryl diradical or an alkylaryl diradical having 1 to 40 carbon atoms and which may contain ether, thio or amine linkages in the main chain;
* denotes a urethane or ureido linkage;
a is at least 1;
A denotes a divalent polymeric radical of the formula: 
xe2x80x83wherein:
each Rz independently denotes an alkyl or fluoro-substituted alkyl group having 1 to 10 carbon atoms which may contain ether linkages between carbon atoms;
mxe2x80x2 is at least 1 and
p is a number which provides a moiety weight of 400 to 10,000;
each Exe2x80x2 independently denotes a polymerizable unsaturated organic radical represented by the formula: 
xe2x80x83wherein:
R23 is hydrogen or methyl;
R24 is hydrogen, an alkyl radical having 1 to 6 carbon atoms, or a xe2x80x94COxe2x80x94Yxe2x80x94R26 radical wherein Y is xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94NHxe2x80x94;
R25 is a divalent alkylene radical having 1 to 10 carbon atoms; R26 is a alkyl radical having 1 to 12 carbon atoms; X denotes xe2x80x94COxe2x80x94 or xe2x80x94OCOxe2x80x94; Z denotes xe2x80x94Oxe2x80x94 or xe2x80x94NHxe2x80x94; Ar denotes an aromatic radical having 6 to 30 carbon atoms; w is 0 to 6; x is 0 or 1; y is 0 or 1; and z is 0 or 1.
A specific urethane monomer is represented by the following: 
wherein m is at least 1 and is preferably 3 or 4, a is at least 1 and preferably is 1, p is a number which provides a moiety weight of 400 to 10,000 and is preferably at least 30, R27 is a diradical of a diisocyanate after removal of the isocyanate group, such as the diradical of isophorone diisocyanate, and each Exe2x80x3 is a group represented by: 
Other silicone-containing monomers include the silicone-containing monomers described in U.S. Pat. Nos. 5,034,461, 5,610,252 and 5,496,871, the disclosures of which are incorporated herein by reference. Other silicone-containing monomers are well-known in the art.
In the case of hydrogels, either the silicone-containing monomer or the hydrophilic monomer may function as a crosslinking agent (a crosslinker being defined as a monomer having multiple polymerizable functionalities) or a separate crosslinker may be employed.
The monomer mixtures include a UV-absorbing agent, defined as an agent that, when incorporated in the final lens, is capable of reducing at least 70% percent of light in the region of 200 to 400 nm, more preferably at least 70% of light in the region of 320 to 400 nm and at least 90% of light in the region of 290 to 320 nm. The invention is suitable for monomer mixtures including any conventional UV absorbing agent. One general class of such agents are non-polymerizable absorbers such as 2,2-drihydoxy-4,4-dimethoxy-benzophenone, and 2,2-dihydoxy-4-methoxy-benzophenone. Preferred, however, are polymerizable UV absorbing agents that include an activated unsaturated group that is reactive with the lens-forming monomers, whereby the UV absorbing agent is copolymerized with the lens-forming monomers. Representative polymerizable UV absorbing materials for such lens applications are described in U.S. Pat. No. 4,304,895 (Loshaek), U.S. Pat. No. 4,528,311 (Beard et al.), U.S. Pat. No. 4,716,234 (Dunks et al.), U.S. Pat. No. 4,719,248 (Bambury et al.), U.S. Pat. No. 3,159,646 (Milionis et al.) and U.S. Pat. No. 3,761,272 (Manneus et al.), the disclosures of which are incorporated herein by reference Specific examples include: benzotriazole-containing monomers such as 2-(2xe2x80x2-hydroxy-5xe2x80x2-methacrylamidophenyl)-5-chlorobenzotriazole, 2-(2xe2x80x2-hydroxy-5xe2x80x2-methacrylamidophenyl)-5-methoxybenzotriazole, 2-(2xe2x80x2-hydroxy-5xe2x80x2-methacryloxypropyl-3xe2x80x2-t-butyl-phenyl)-5-chlorobenzotriazole, 2-(2xe2x80x2-hydroxy-5xe2x80x2-methacryloxyethylphenyl)benzotriazole, 2-(2xe2x80x2-hydroxy-5xe2x80x2-methacryloxypropylphenyl)benzotriazole; and the polymerizable benzophenones described in U.S. Pat. No. 4,304,895.
The monomer mixtures may also include a tinting agent, defined as an agent that, when incorporated in the final lens, imparts some degree of color to the lens. The invention is applicable to conventional tinting agents known in the art, including non-polymerizable agents, or polyrnerizable agents that include an activated unsaturated group that is reactive with the lens-forming monomers. One preferred example of this latter class is the compound 1,4-bis(4-(2-methacryloxyethyl)phenylamino)anthraquinone, a blue visibility-tinting agent disclosed in U.S. Pat. No. 4,997,897.
As mentioned, photopolymerization of monomer mixtures to form lenses by UV curing has proved very effective, however, for lenses including a UV absorbing agent, ineffective or uneven curing is encountered since this agent absorbs UV light. The invention provides a method whereby lenses including both a UV absorbing agent and a tinting agent can be effectively photopolymerized by free radical polymerization.
More specifically, it was found that use of an initiator that includes a phosphine oxide moiety permitted satisfactory curing of monomer mixtures by photopolymerization to form lenses. Accordingly, it is preferred that the initial monomer mixtures include a phosphine oxide-containing initiator. The phosphine oxide moiety may be represented by the formula: 
Preferred initiators include the following phosphine oxide-containing radical: 
where n is zero or one, and preferably one.
Representative compounds with this phosphine oxide-containing moiety are of the formula: 
wherein Ar and Arxe2x80x2 are independently an optionally substituted aromatic radical, and R is an alkyl or optionally substituted aromatic radical, and n is zero or one and preferably one. Specific examples of such phosphine oxide-containing compounds include: bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (TMBPPO); bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (DMBAPO); 2,4,6-trimethylbenzyldiphenyl phosphine oxide; and 2,4,6-trimethylbenzyoyl diphenylphosphine oxide (MAPO). Commercially available initiator systems with these compounds include: Irgacure-819(trademark) initiator, based on TMBPPO (Ciba Specialty Chemicals); Irgacure-1700(trademark) initiator, including DMBAPO at about 25 wt % (Ciba Specialty Chemicals); Irgacure-1800(trademark) initiator, including DMBAPO at about 25 wt % (Ciba Specialty Chemicals); MAPO (Ciba Specialty Chemicals); and Lucirin TPO(trademark) initiator, based on 2,4,6-trimethylbenzyldiphenyl phosphine oxide (BASF).
Generally, the monomer mixtures is charged to a mold, and then subjected to light to effect curing of the monomer mixture in the mold. Various processes are known for curing a monomeric mixture in the production of contact lenses, including spincasting and static casting. Spincasting methods involve charging the monomer mixture to a mold, and spinning the mold in a controlled manner while exposing the monomer mixture to light. Static casting methods involve charging the monomer mixture between two mold sections, one mold section shaped to form the anterior lens surface and the other mold section shaped to form the posterior lens surface, and curing the monomer mixture by exposure to light. Such methods are described in U.S. Pat. Nos. 3,408,429, 3,660,545, 4,113,224, 4,197,266, and 5,271,875.
For the present invention, any light source may be used so long as it provides light in the visible region of the spectrum, and especially 400 to 500 nm. It is noted, however, that in some cases it may be desirable to filter out light in the ultraviolet region of the spectrum, especially light in the region of 300 to 400 nm; in some cases, exposure to light in this region may lead to undesired xe2x80x9cwarpingxe2x80x9d of the lens or xe2x80x9ccurlingxe2x80x9d at edges of the lens. Accordingly, although the light source does not need to provide light exclusively in the visible region of the spectrum, according to a distinct preferred embodiment the monomer mixture is exposed to light predominantly in the visible region of the spectrum. This may be accomplished either by selection of an appropriate light source relatively specific to visible light, or by using a light source providing a broad spectrum of light and filtering out UV radiation impinging on the monomer mixture.
The following examples illustrate various preferred embodiments. The following nomenclature is used in the description of experimental procedures:
F2D20xe2x80x94a polysiloxanediol-based fumarate prepolymer of Formula (IIc) endcapped with t-butylamine (derived from a polydimethylsiloxanediol, fumaryl chloride and t-butylamine according to U.S. Pat. No. 5,420,324) and described more fully in Synthesis C below.
ID2S4Hxe2x80x94a polyurethane-based prepolymer endcapped with 2-methacryloxyethyl (derived from isophorone diisocyanate, diethylene glycol, a polydimethylsiloxanediol, and 2-hydroxyethyl methacrylate according to U.S. Pat. No. 5,034,561) and described more fully in Synthesis A below.
ID3S4Hxe2x80x94a polyurethane-based prepolymer endcapped with 2-methacryloxyethyl (derived from isophorone diisocyanate, diethylene glycol, a polydimethylsiloxanediol, and 2-hydroxyethyl methacrylate according to U.S. Pat. No. 5,034,561) and described more fully in Synthesis B below.
TRISxe2x80x943-methacryloxypropyl tris(trimethylsiloxy)silane
DMAxe2x80x94N,N-dimethylacrylamide
IMVTxe2x80x941,4-bis(4-(2-methacryloxyethyl)phenylamino)anthraquinone (Example 1i of U.S. Pat. No. 4,997,897), a blue visibility-tinting agent
UV Agentxe2x80x942-(2-hydroxy-5-methacrylamidophenyl)-5-methoxybenzotriazole (Example 4 of U.S. Pat. No. 4,719,248)
TXNxe2x80x94thioxoanthen-9-one
MDEAxe2x80x94N-methyldiethanolamine
Darocur-1173(trademark)xe2x80x94a commercial acetophenone-based initiator (Ciba Specialty Chemical), based on 2-hydroxy-2-methyl-phenylpropan-1-one
Irgacure-184(trademark)xe2x80x94(I-184) a commercial acetophenone-based initiator (Ciba Specialty Chemical), based on 1-hydroxycyclohexyl phenyl ketone
Irgacure-784(trademark)xe2x80x94(I-784) a commercial titanocene-based initiator (Ciba Specialty Chemical)
Irgacure-819(trademark)xe2x80x94(I-819) a commercial initiator based on bis(2,4,6-trimethylbenzoyl) phenylphosphine oxide (Ciba Specialty Chemical),
Irgacure-1700(trademark)xe2x80x94(I-1700) a commercial initiator based on bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (about 25 wt %) and the Darocur-1173(trademark) acetophenone-based initiator (Ciba Specialty Chemical)
Irgacure-1800(trademark)xe2x80x94(I-1800) a commercial initiator based on bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (about 25 wt %) and the Irgacure-184(trademark) acetophenone-based initiator (Ciba Specialty Chemical).
Synthesis A
Preparation of a Polydimethylsiloxane-based Polyurethane Polymerxe2x80x94(ID2S4H)
A dry 3-neck, 1000 mL round bottom flask was connected to a nitrogen inlet tube and a reflux condenser linked. Then, isophorone (16.916 g, 0.0761 mole), diethylene glycol (4.038 g, 0.0380 mole), dibutyl tin dilaurate (0.383 g) and 140 mL of methylene chloride were added into the flask all at once and the contents were refluxed. After 16 hours, the amount of isocyanate was determined and decreased to 47.0% by titration. Then xcex1, xcfx89-bis(4-hydroxybutyl)polydimethylsiloxane (102.56 g, 0.02536 mole) was added into the flask. Refluxing was continued for 33 hours, and the amount of isocyanate was decreased to 14.1% of the original by titration. The contents were then cooled to ambient temperature. 2-Hydroxyethyl methacrylate (2.2928 g) and 1,1xe2x80x2-bi-2-phenol (0.0129 g) were then added and the contents were stirred at ambient until isocyanate peak at 2267 cmxe2x88x921 disappeared from IR spectrum of the product. The solvent was then stripped under reduced pressure to yield the product.
Synthesis B
Preparation of a Polydimethylsiloxane-based Polyurethane polymerxe2x80x94(ID3S4H)
The procedure in Example 1 was followed except that the molar ratios of ingredients was varied. Specially, the amounts of ingredients in the early synthesis step were: isophorone diisocyanate (10.425 g, 0.04799 mole) and diethylene glycol (2.5469 g, 0.024 mole); and in the subsequent synthesis step, xcex1, xcfx89-bis(4-hydroxybutyl)polydimethylsiloxane (74.22g, 0.01799 mole), and 2-hydroxyethyl methacrylate (1.8376 g).
Synthesis C
Preparation of a Polydimethylsiloxane-based Fumarate Prepolymer End-capped with t-butylamine (F2D20)
A 3-neck, 500 mL round bottom flask was connected to a nitrogen inlet tube and a reflux condenser was linked in series to a potassium hydroxide tube and a sodium hydroxide solution. Fumaryl chloride (12.56 g, 0.082 mole), xcex1, xcfx89-bis(4-hydroxybutyl) polydimethylsiloxane of Mn 1595 (59.81 g, 0.0375 mole) and 250 mL of anhydrous methylene chloride were added to the flask. The contents were refluxed under nitrogen purging. After 18 hours, it was found that the OH groups were gone by analyzing an aliquot by high resolution NMR. The unreacted fumaryl chloride and the solvent were stripped under vacuum. Next, 50 mL of methylene chloride was added and the mixture cooled to 0-5xc2x0 C. Then, t-butylamine (11.67 g, 0.1595 mole) in 250mL of methylene chloride was added slowly to maintain a low temperature. The mixture was kept at room temperature while stirring overnight. The ammonium salt was filtered and the mixture washed with aqueous sodium bicarbonate two times and then with water until neutral. The product was dried with magnesium sulfate and the methylene chloride was stripped. This crude product (64.5 g) was then redissolved in 130 mL of methylene chloride and passed through a silica gel column. The first 195 mL was collected then the column was eluted with 65 mL of methylene chloride. The final combined 260 mL eluent was rotevaporated and vacuum dried at 80xc2x0 C. to give to give 62.3 g product. The prepolymer was characterized by IR, NMR and size exclusion chromatography (SEC). IR (cmxe2x88x921):3325, 2962, 1727, 1644, 1541, 1456, 1412, 1365, 1297, 1257, 1222, 1159, 1010, 963 and 786; 200 Mhz H-NMR (ppm) 0.00, 0.56 (t), 1.39 (s,) 1.40 (m) 1.69 (m), 4.17 (t), 5.70 (broad), 6.77 (m); SEC (by polystyrene standard) gave Mn of 2593, Mw 3,887; polydispersity 1.50.
Preparation of Monomer Mixtures
A first series of monomer mixtures, suitable for providing silicone hydrogel contact lenses, were prepared by mixing the following components:
Mixture 1Axe2x80x94F2D2 (20 parts by weight (pbw)); TRIS (40 pbw), DMA (40 pbw); hexanol solvent (20 pbw); and Darocur-1173 initiator (0.5 pbw)
Mixture 1Bxe2x80x94F2D20 (20 parts by weight (pbw)); TRIS (40 pbw), DMA (40 pbw); hexanol solvent (20 pbw); and UVAgent (0.5 pbw)
Mixture 1Cxe2x80x94F2D20 (20 parts by weight (pbw)); TRIS (40 pbw), DMA (40 pbw); hexanol solvent (20 pbw); UVAgent (0.5 pbw); and IMVT (150 ppm)
A second series of monomer mixtures, suitable for providing silicone hydrogel contact lenses, were prepared by mixing the following components:
Mixture 2Axe2x80x94ID2S4H (50 parts by weight (pbw)); TRIS (20 pbw), DMA (30 pbw); hexanol solvent (20 pbw); and Darocur-1173 initiator (0.5 pbw)
Mixture 2Bxe2x80x94ID2S4H (50 parts by weight (pbw)); TRIS (20 pbw), DMA (30 pbw); hexanol solvent (20 pbw); and UVAgent (0.5 pbw)
Mixture 2Cxe2x80x94ID2S4H (50 parts by weight (pbw)); TRIS (20 pbw), DMA (30 pbw); hexanol solvent (20 pbw); UVAgent (0.5 pbw); and IMVT (150 ppm).
General Preparation of Hydrogel Films
Monomer mixtures were cured into films by placing the mixture between two silane-treated glass plates and exposing to a light source for 1 hour. Cured films were released from the plates and extracted with isopropanol, then heated in boiling water. Films were equilibrated in borate buffered saline before characterization.